Lift trucks are designed in a variety of configurations to perform a variety of tasks. A lift truck traveling throughout a facility may encounter debris on the floor and uneven floor surfaces. These can take the form of expansion joints, cracks in the floor surface or man-made objects such as ramps going between buildings or into tractor trailers. Tire irregularities and/or the floor can also cause periodic vibration that can be transmitted throughout the frame of the truck.
When a lift truck is traveling fast, one or more wheels strike an edge of the uneven surface harder than when the truck is traveling slowly. The energy from this motion is transmitted throughout the truck in the form of shock waves. Referring to FIG. 1, the resultant forces may be transmitted throughout a truck 10 in any of the three axes, including the X-axis 12, the Y-axis 14, and the Z-axis 16, and may be felt by the operator (not shown) creating a sense of discomfort. The truck 10 may include a tractor unit 17 and vertically movable forks 19 mounted relative to the tractor unit, the movable forks being vertically movable between an upper position and a lower position. If there is no operator, such as when the truck is being remotely controlled, the resultant forces may still have negative effects because equipment 18 on the truck may be rendered less effective. If there is equipment 18 on the truck, such as sensitive sensory equipment, the quality of the data from the equipment 18 may diminish because of the resultant forces and resultant truck reaction.
When a lift truck strikes a floor condition that affects only one side, such as when only one wheel, such as caster wheel 20, strikes a raised crack in the floor, and that side is forced up, the resultant motion is commonly called roll, and is shown as movement about the X-axis 12. The effect of roll causes the entire truck to temporarily move or tilt to one side (to the side of the truck with the wheel that did not strike the crack), and any sensory equipment mounted on the truck will also be directed to the same side. Equipment 18, such as a sensor mounted several feet away from the original point of movement, (the caster wheel 20), will have its reaction exaggerated. The sudden movement caused by the floor condition can diminish the effectiveness and/or accuracy of the sensory equipment and may necessitate that the truck be operated at slower speeds to reduce the effects of the floor conditions. Slower operating speeds may equate to an undesirable reduction in overall equipment productivity.
Referring to FIG. 2, a variety of lift truck configurations use spring loaded casters 24, including for example pallet trucks and stacker trucks, that have a center traction wheel 26 spaced between the two spring loaded casters 24. The spring loaded casters allow for driving over rough surfaces or floors 28 while still maintaining good contact force for the traction wheel 26. This contact force is important because acceleration, braking, and control are mainly achieved through the traction wheel, so the wheel 26 should maintain floor contact with enough force to control the truck motion. Typically, the casters are adjusted to find an optimum operation between traction wheel slippage and the truck rocking or tilting between both casters.
The casters 24 can be adjusted by adding shims 30 to push down harder on the floor, thus raising the truck slightly, or shims can be removed to make caster springs 32 push less, thus slightly lowering the truck. Typically, the caster springs 32 themselves are not adjusted. The purpose of the shim 30 is to set how much of the vehicle weight is on the traction wheel 26 versus how much is on the spring loaded casters 24. Without the flexible spring loaded casters and the ability to adjust the casters with shims, the casters may end up carrying most of the weight and the traction wheel may slip due to not enough contact force, or the traction wheel may take most of the weight, causing the truck to slightly tilt to one caster or the other.
Caster adjustment may be time consuming, and may include jacking up the truck, estimating how big a shim to install, and then seeing if the shim was too big or too little. In this configuration, there is no dynamic adjustment of the spring force while the truck is in motion.
Spring only casters can range from hard, with a high spring constant, to soft, with a low spring constant. A softer caster tolerates a rougher floor, but also lets the truck tilt on turns and shifts in the load or the operator position. A hard caster works well as long as the floor is completely flat. Conversely, the operator may sense rough floor conditions and objects on the floor, or cracks in the floor may effect the truck as the caster rolls over them.
Other varieties of lift truck configurations use spring loaded casters and include a known shock absorber 34 for damping. The addition of damping allows for softer springs, but still reduces the oscillation of rocking on a rough floor. Nevertheless, when one caster hits a large bump on the floor, the damper responds to the high speed motion of the caster by generating considerable force and may tilt the truck because the damper force is a function of caster motion, not truck roll. When the damper reacts in this way, it diminishes the advantages of softer springs.
Other varieties of lift truck configurations use a sway bar or torsion bar 36 between casters 24. Much like the spring and shock absorber configuration described above, with the inclusion of a torsion bar 36, the rough floor is averaged out so small random bumps don't tilt the truck. When tilting to one side, it automatically reduces the spring force on the other, which may stop the tilt. But also like the spring and shock absorber configuration, with a torsion bar, one caster hitting debris may raise the caster on the other side. On the contact side, the caster will push up while at the same time the caster on the other side is being pulled up by the torsion bar. So, in some cases, the torsion bar may actually induce a tilt in the truck.
Referring to FIG. 3, still other varieties of lift truck configurations use fixed casters 38 and a suspended traction wheel 40. This configuration lets the suspension springs 42 provide enough force to keep the traction wheel in contact with the floor, and is more prevalent with very flat floors. On rough floors, operators of a truck with this configuration are known to feel oscillations and the truck may tilt on most every bump. Also, the effect of hitting an object with one caster may cause a significant contact and tilt.
The prior methods suffer from not monitoring the orientation of the truck in one or more of the three axis of motion. For example, the casters only need to put out a force when the truck is moving away from horizontal, or roll, in the X-axis 12. If the truck is horizontal, or not changing quickly from horizontal, then the caster spring could be very soft. Yet, all these existing solutions respond to vertical motion of the caster wheel regardless of whether it is tilting the truck or not.
At best, the prior methods only improve the tradeoff between soft springs and the truck rolling versus hard springs and truck dampers that limit the performance of the spring only caster configurations. Another disadvantage of these previously used methods has been that the force or motion created is fixed. Even though variables like speed, mass and direction of motion are changing constantly as the truck is used, the compensating forces from a spring or shock are fixed, having been calculated from average or nominal values. Therefore only a narrow range of motion can be effectively addressed or mitigated.
If the motion of the caster wheel can be mitigated or even cancelled, the truck would then be capable of traveling faster without the potential damage to components or loss or degradation of truck data, along with a more comfortable ride for the operator. A more stable mounting platform for sensitive sensory components also improves the quality of data produced, allowing greater flexibility in the use of the truck in either automatic or manual modes.
What is needed is a lift truck configured to retain desirable features of flexible casters and yet to add more stability control to the lift truck.